1. Technical Field
The present invention relates to a piezoelectric drive device and a driving method thereof, and a robot and a driving method thereof.
2. Related Art
Since a piezoelectric actuator (piezoelectric drive device), which vibrates a piezoelectric material and drives a driven body, does not require a magnet or a coil, the piezoelectric actuator is used in various fields (for example, JP-A-2004-320979). A basic configuration of the piezoelectric drive device is as follows. That is, four piezoelectric elements are configured so as to be disposed 2×2 on each of two surfaces of a reinforcing plate, and a total of eight piezoelectric elements are provided on both sides of the reinforcing plate. Each piezoelectric element is a unit in which the piezoelectric material is interposed between two electrodes, and the reinforcing plate is used as one electrode of the piezoelectric element. A protrusion portion, which comes into contact with a rotor serving as the driven body to rotate the rotor, is provided on one end of the reinforcing plate. When an alternating voltage is applied to two piezoelectric elements which are diagonally disposed among four piezoelectric elements, the two piezoelectric elements perform expansion and contraction movements. Accordingly, the protrusion portion of the reinforcing plate performs a reciprocating movement or an elliptical movement. In addition, the rotor serving as the driven body rotates in a predetermined rotation direction according to the reciprocating movement or the elliptical movement of the protrusion portion of the reinforcing plate. In addition, by switching the application target of the alternating voltage from the two piezoelectric elements to the other two piezoelectric elements, it is possible to rotate the rotor in a reverse direction.
In an operating device (robot or the like) to which the piezoelectric drive device is applied, in a state where a movement of an operating portion stops, the protrusion portion presses the driven body. Accordingly, a static friction force generated between the protrusion portion and the driven body becomes a holding force or holding torque for holding the operating portion connected to the driven body in a stationary state. Therefore, if an external force exceeding the holding force is applied to the operating portion, the stationary state of the driven body cannot be held, and the operating portion moves.
In addition, during the operation of the operating portion, the piezoelectric drive device generates output torque exceeding the holding torque by a great pressing force which is generated by the movement (elliptical movement or the like) of the protrusion portion. Accordingly, the operating portion connected to the driven body is operated. In addition, when the movement of the operating portion is decelerated, apparent output torque increases due to an inertial force. Since output torque greater than the holding torque is generated, it is possible to move the operating portion by load torque greater than the holding force. However, there is a problem that the moment the movement of the operating portion stops, the operating portion moves in a direction opposite to the movement direction immediately before the operating portion stops, and the operating portion cannot be stopped at a desired position.
In addition, during the operation of the operating portion, in a case where the holding torque and the output torque antagonize each other, if slight sliding is temporarily generated between the protrusion portion and the driven body, since the operating portion is changed from a stationary friction state to a dynamic friction state and the friction force decreases, there is a problem that the operating portion continuously slides without being stopped and cannot be stopped. Accordingly, a technology capable of stopping the operating portion at a desired position after the movement of the operating portion starts and moving the piezoelectric drive device so as to hold the stationary state at the desired position is required.